Combine thyme, garlic, 1 teaspoon salt, and pepper; coat beef tenderloin with mixture. Heat a large ovenproof skillet with cooking spray over medium-high heat. Add beef; cook 2 minutes on 1 side until browned. Turn beef over.
Transfer skillet to oven; bake at 350° for 25-30 minutes or until internal temperature reaches 125°. Transfer beef to a carving board; tent with foil and let stand 10 minutes (internal temperature will rise to 130° for medium-rare).
Meanwhile, melt butter in same skillet over medium heat. Add mushrooms and remaining salt; cook 5 minutes, stirring frequently. Add broth and wine; simmer 8 minutes. Combine cornstarch and water; mix well. Stir into sauce; simmer 2 minutes or until slightly thick.
Cut beef crosswise into 16 slices; transfer to serving plates and top with mushroom sauce.
Our expert's wine pick: The beef is perfect with Montevina Terra d'Oro Zinfandel ($18) from California. Its spice echoes the peppery thyme flavor. Tip: Most guests drink about 2 glasses of wine, and there are 4 glasses per bottle. So if 8 people are coming, you'll need at least 4 bottles of wine. Get a few extra, just in case. --Andrea Immer Robinson